1. Field of the Invention
The present invention relates to rangefinders, and more particularly to pulse-echo radar rangefinders that are powered by an industrial two-wire current signaling loop.
2. Description of Related Art
Control instrumentation for industrial processes traditionally use a two-wire or a four-wire interface between a sensor (called a xe2x80x9ctransmitterxe2x80x9d) and a controller or data processor. The four-wire arrangement uses two wires for power, and two other wires for signaling using a current-loop format. Carrier-based digital modulation may also be impressed on the two-wire current loop, such as the HART(copyright) Protocol, for communication and control.
The two-wire interface uses only two wires for both conveying power to the transmitter and conveying proportional analog data from the transmitter. The proportional analog signal most often conforms to a 4-20 mA standard that specifies 4 mA to power the transmitter and 0-16 mA to indicate an analog value. The two-wire loop is preferred due to its lower cost, its lower explosion hazard and a large installed base of two-wire links at industrial sites. However, two-wire operation poses severe power constraints on the transmitter: a few tens of milliwatts.
Loop-powered transmitters date back to at least 1977. For example, U.S. Pat. No. 4,016,763 to Grindheim, 1977, discloses a resistance bridge in a loop-powered circuit. U.S. Pat. No. 4,242,665 to Mate, 1980, discloses a two-wire circuit that achieves low average power using a high power sensor operated with duty-cycled power. An ultrasonic rangefinder operating on loop power was presented in xe2x80x9cA Two-Wire Ultrasonic Level Meter with Piezoelectric Polymer-Film Sensorxe2x80x9d by Owada et al, Proceedings of the ISA/88 International Conference and Exhibit, Vol. 43, Part 3, 1988. Thus, by 1988 the details of loop-powered pulse-echo ultrasonic rangefinders were published. However, pioneering work on loop-powered pulse-echo radar rangefinders did not commence until the 1990""s.
A motion sensor using a high power radar that achieves low average power by using duty-cycled power was disclosed in U.S. Pat. No. 4,131,889 to Gray, 1978. While Gray did not power his radar from an industrial loop, it would have been evident to do so by 1978 in view of Grindheim and 1980 in view of Mate. Nonetheless, the resulting loop-powered radar based on the Gray patent would not be capable of measuring range, or more particularly, tank levels since Gray""s radar only detected motion.
FIG. 1a schematically depicts the first known low power radar to operate on a two-wire loop. It was prototyped in 1993 and disclosed in U.S. Pat. No. 5,465,094 to McEwan, 1995. Although the loop was scaled for automotive use and signaled a discrete current level, it would have been apparent to a practitioner in 1993 to adapt it to the 4-20 mA industrial standard. In operation, a low current is received over two wires 20 and stored in a power store element 14 such as a capacitor, which provides power to voltage regulator 12, which powers radar 10. A low power radar 10 (also known as micropower impulse radar, or MIR) detects motion and responsively operates a shorting switch 16 to increase the current on the 2-wire interface 20. During the time switch 16 is closed, the voltage on wire pair 20 drops to zero, so radar 10 operates on stored power from power store element 14.
In early 1994 a low power MIR impulse radar rangefinder was prototyped, forming the basis for U.S. Pat. Nos. 5,774,091; 5,757,320; and 5,805,110, all to McEwan, 1998. While not specifically set up for loop operation, a practitioner could readily have adapted the impulse radar for 4-20 mA two-wire loop operation in early 1994.
FIG. 1b schematically depicts a loop-powered impulse radar rangefinder as disclosed in U.S. Pat. No. 5,672,975 to Kielb et al, 1997, assigned to Rosemount, Inc. A two-wire interface 20 provides power to voltage regulator 12, which in turn powers impulse radar 11. A measurement circuit 22 initiates transmissions and starts a range measurement timer. Impulse radar 11 ends the range measurement upon receipt of an echo. The measurement circuit 22 provides an output to a 0-16 mA proportional analog current source 18 to signal the measured range across two-wire loop 20. The entire apparatus must draw 4 mA from two-wire loop 20 so the total current spans 4-20 mA.
FIG. 1c schematically depicts a loop-powered impulse radar rangefinder disclosed in U.S. Pat. No. 6,014,100 to Fehrenbach et al, 2000, assigned to Vega Grieshaber, AG. A high power radar 13 is operated with duty-cycled power to achieve low average power. Power store element 14 provides high current surges to high power radar 13 and averages the high power surges with inactive periods drawing little or no power so the current draw from regulator 12 is low. This duty-cycled power technique appears to be similar to that described by the Gray ""889 patent in 1978. The advantage to using a high power radar design is that the analog circuitry can operate with lower impedances for better moisture immunity and stability, lower cost, and less complexity. Further, high frequency transistors require about 10 mA bias current, which alone could exceed the available power. Aside from duty-cycled power, high power radar 13 appears to be similar in operation to low power radar 11, as stated by Fehrenbach et al, xe2x80x9csignal generation and processing during and after measurements are as described, for instance, in U.S. Pat. No. 5,672,975.xe2x80x9d Having provided no other technical details on radar 13, it can only be assumed that it is an impulse radar having similar timing to that of impulse radar 11.
The prior loop-powered rangefinding radars, as depicted in FIGS. 1b and 1c, are impulse radars. Step generator 76 in FIG. 2 of the ""975 patent indicates its impulse nature. The output of step generator 76 is differentiated into an impulse by antenna 18xe2x80x94all antennas, including antenna 18 in the ""975 patent, differentiate a step input into a radiated impulse. Microwave circulators, such as circulator 78 in the ""975 patent, pass an ultrawideband spectrum and offer essentially no bandlimiting action, so antenna 18 defines the emission spectrum. Thus, radars 11, 13 are damped wave devices, and most likely radiate over a broad spectral region, such as 1-5 GHz, or with a resonant horn antenna, perhaps 4-8 GHz. Radars 11, 13 pose a serious regulatory limitation: damped wave emitters have been prohibited in the U.S. and internationally since 1934. An impulse radar spectrum crosses numerous restricted bands, particularly those used by GPS equipment and aviation safety radar. Impulse radars 11, 13 cannot receive FCC equipment authorization under current regulations and therefore have little or no commercial value.
The FCC strictly prohibits intentional radiation in the restricted bands, no matter how weak. Accordingly, adding a filter to the output of an impulse radar to limit spectral radiation in the restricted bands may be viewed in the same light as adding an attenuator to the outputxe2x80x94it does not change the intent of the emissions. Similarly, operating an impulse radar in a tank may be viewed as adding an attenuator to the output of a radar having intentional radiation in the restricted bands.
FCC prohibitions notwithstanding, the impulse radar described in the ""975 patent (and by incorporation, the ""100 patent) appears to have at least four deficiencies which would block practical implementation. First, the ""975 specification cites the receive clock frequency f2=f1 +xcex94f, where f1 is the transmit clock and f2 is the receive clock, xcex94f being a 10-40 Hz offset. As is well known in this type of slipped-phase clock system, the frequency relation should be f2=f1xe2x88x92xcex94f. The effect of this error is to make the sampled equivalent time output of receiver 70 appear to run backwards, so an echo appears before a pulse is transmitted. There is no suggestion of how to measure or process time-reversed signals.
Second, the ""975 specification states xe2x80x9c. . . the receive and transmit circuits in circuitry 70 are electrically isolated from each other. This is important so that transmit pulses are not incorrectly detected by the receiver as the echo pulse.xe2x80x9d A practitioner would know that the transmit-receive isolation provided by a realizable circulator is on the order of 20-30 dB. Since practical echo signals are 40-120 dB weaker than the transmit pulse, the transmit pulse will always be much stronger than any echo pulse and therefore the transmit pulses will always be xe2x80x9cincorrectly detectedxe2x80x9d as echo pulses. Accordingly, the radar described in the ""975 patent will always register zero range.
Third, a critical element is missing in radar 70 of the ""975 patent. It is stated that the xe2x80x9cmeasurement circuitry initiates the transmitting of the microwave signal and determines product height based upon the reflected signal received by the receiver.xe2x80x9d The missing element is a phase detector or other means to synchronize the initiation of timing measurements. Range timing measurements must begin when clock 1 and clock 2 are in phase coincidence (the transmit time) and continue until clock 2 slips in phase to align with an echo pulse (the receive time). While the echo pulse phase alignment is provided by pulse detection in the receiver, there is no transmit phase alignment detector, thereby rendering any transmit-to-receive time measurement meaningless.
Fourth, the ""975 patent provides no details on its impulse receiver, which must have ultra-wide bandwidth while consuming very little power. There are several references to MIR, including a low power MIR receiver, U.S. Pat. No. 5,345,471 to McEwan. The MIR receiver receives impulses and outputs an integrated signal. It is not a xe2x80x9cpulses-in, pulses-outxe2x80x9d receiver, as was clearly established by a reexamination (certificate B1 U.S. Pat. No. 5,361,070). The ""975 patent states xe2x80x9cthe output of impulse receiver 80 is a series of impulses.xe2x80x9d Pulse-by-pulse operation is further indicated by analog to digital converter 82 xe2x80x9csince a sample must be taken after every transmit pulse . . . xe2x80x9d A low power pulse-by-pulse receiver is not disclosed in the specification or the references. There is no known low power xe2x80x9cpulse-by-pulsexe2x80x9d ultrawideband receiver that could be used in the ""975 patent, and the ""975 patent does not disclose any details thereof.
Assuming inventive fixes could be added to the ""975 patent to overcome these four deficiencies, the resulting system would have serious timing inaccuracies since the transmit-to-receive time measurement includes not only the desired echo delay time, but also the propagation delays through the entire transmitter and receiver. Commercial tank level radars require stability on the order of 1 cm or 66 ps or better. The delay variation in even one logic gate or transistor can exceed 66 ps, not to mention an entire transmitter and receiver. Means to address commercial accuracy requirements are not disclosed in the ""975 patent.
Precision analog signaling over a 4-20 mA loop involves maintaining the transmitter power supply current at exactly 4 mA and then adding 0-16 mA for the analog signal. The prior art radars of FIGS. 1b and 1c do not disclose a means to precisely regulate the 4 mA transmitter power. They appear to rely on the transmitter itself to somehow draw exactly 4 mA. Presumably, a load trimmer could be adjusted to obtain exactly 4 mA power draw. Overall accuracy would then be limited by drift in transmitter current, perhaps+/xe2x88x920.2 mA, which would degrade the 0-6 mA signaling accuracy to about 1% of full scale.
A radar rangefinder employing FCC-compatible pulsed RF emissions with two antennas is described in U.S. Pat. No. 6,137,438, xe2x80x9cPrecision Short-Range Pulse-Echo Systems with Automatic Pulse Detectors,xe2x80x9d to McEwan, 2000. The ""504 patent does not suggest loop powered operation, although it could be suitably configured by a practitioner. Operation with a single antenna (rather than separate transmit and receive antennas) allows operation through a smaller tank openingxe2x80x94a competitive and cost saving feature. A single antenna pulse-echo radar employing harmonic techniques is disclosed in U.S. patent application Ser. No. 09/416,835, Homodyne Swept-Range Radar,xe2x80x9d to McEwan. This application does not suggest loop-powered operation, although a practitioner could configure the apparatus for loop-powered operation. Fundamental-mode receiver operation, or operation with a single antenna connected via a cable is not discussed.
In summary, the prior art does not suggest a loop-powered, pulse-echo radar range finder that (1) is FCC compliant (i.e., non-impulse), (2) has a 4 mA (or other current) regulator, and (3) employs an accurate, externally referenced measurement system using a single antenna without a microwave circulator.
The present invention is a short-range radar transceiver that uses the same pulsed-RF oscillator for both a transmit oscillator and a swept-in-time receive local oscillator. The dual function use of one oscillator eliminates the need for two microwave oscillators and facilitates operation with only one antenna for both transmit and receive functions. Further, it assures optimal operation since there are no longer two oscillators that can go out of tune with each other (in a similar radar having two RF oscillators, both oscillators must be tuned to the same frequency). U.S. patent application Ser. No. 09/416,835, Homodyne Swept-Range Radar,xe2x80x9d to McEwan describes a homodyne radar using a single harmonic oscillator and a receive harmonic sampler. The present invention employs a non-harmonic fundamental frequency transmit oscillator and non-harmonic fundamental frequency receive sampler. Unlike the harmonic radar of the ""835 Application, the present invention employs a fundamental frequency homodyne system that avoids the problem of injection locking on strong echo pulses (an effect described in the ""835 Application) by operating at lower microwave frequencies where the pulsed RF oscillator can be strongly injection-locked to its drive pulses, and by loosely coupling the RF oscillator to the antenna through a transmission line or waveguide.
A key departure from the homodyne radar of application Ser. No. 09/416,835 is the use of a coaxial cable (or waveguide) to connect to the antenna. This arrangement produces a reflection at the coax/antenna interface that is used as a measurement reference plane. This differs from prior art pulse-echo radars that either (1) use the transmit main bang as a measurement reference (e.g., U.S. Pat. No. 6,137,438), or (2) employ a circulator in a poor effort to limit the main bang coupling into the receiver for the purpose of preventing false receiver triggering (e.g., U.S. Pat. No. 5,672,975). In contrast, the present invention generally uses a time window to exclude the main bang coupling from the receiver output.
The reflection from the antenna forms a timing fiducial pulse and thus a measurement reference plane. The time between the fiducial reflection and a target echo defines the distance between the antenna reference plane and a target, e.g., a material level in a tank. The fiducial scheme improves the measurement accuracy while being an advantageous physical arrangement since the antenna is usually separated from the electronics package. That is, the antenna is inside the tank and the radar electronics are outside the tank, so a transmission line or waveguide must connect them. A fiducial reference scheme has been used with FMCW radars, as seen in U.S. Pat. No. 4,847,623, xe2x80x9cRadar Tank Gauge""xe2x80x9d , to Jean et al, 1989, and in TDR systems such as U.S. Pat. No. 5,609,059 xe2x80x9cElectronic Multi-Purpose Material Level Sensor,xe2x80x9d to McEwan, 1997. However, there is no prior suggestion of how to implement the fiducial scheme with a pulsed-RF pulse-echo radar.
A shunt current regulator is provided to regulate the total current drawn by the apparatus to precisely 4.00 mA (in a 4-20 mA loop for example) regardless of how much current is drawn by the radar (but always less than 4 mA). The current regulator improves the combined accuracy of the 0-16 mA signal current added to the 4.00 mA and it effectively limits drift with time and temperature.
A new sensitivity time control (STC) is provided that is simpler than the digital STC described in U.S. Pat. No. 6,031,421, xe2x80x9cControlled Gain Amplifierxe2x80x9d to McEwan, 2000, or the FET-based STC disclosed in U.S. Pat. No. 5,805,110, xe2x80x9cImpulse Radar With Swept-Range Gate,xe2x80x9d to McEwan, 1998. The new STC employs a current-controlled silicon bipolar transconductance element that increases receiver gain proportionally to the radar gate range.
The emission spectrum from a short-pulse RF oscillator is very broad (often greater than 1 GHz) and appears very low in amplitude on a spectrum analyzer of limited bandwidth, e.g., 1 MHz bandwidth, as preferred in FCC tests. Consequently, narrowband, RF marker pulses are interleaved with the short, coherent RF ranging pulses to produce a highly visible spectrum with an identifiable peak, i.e., carrier frequency. However, the marker pulses may create spurious echoes. To avert this possibility, the marker pulses are randomized in phase so their echoes average to zero in the receiver. Alternatively, the marker pulse transmissions can be time-locked to the range gate to produce a zero beat receiver output that is easily rejected with a simple highpass filter. In either case, the marker pulses produce no receiver output when there is no jamming. When there is jamming, the marker pulses mix with the jamming to produce a detectable output from the receiver, causing jam detection circuitry to output an alarm or control signal.
The present invention is a precision radar rangefinder that can be used in radars for many applications, e.g., tank level measurement, including 0.01% accurate custody transfer measurement; industrial and robotic controls; vehicle backup warning and collision-detection radars; and general rangefinding applications. Since the present invention is phase coherent, microwave holograms can be formed using techniques known in the art, where the customary holographic reference beam is conveniently replaced by the internal phase coherent timing of the present invention. In addition to these features the total power consumption is about 10 mW, sufficiently low for 4-20 mA loop-powered applications.
A primary object of the present invention is to provide a precision, low cost, FCC compatible pulse echo radar ranging system having a single antenna.
A further object of the present invention is to provide a high accuracy fiducial reference with a physical embodiment suited to the needs of tank level measurement.
Yet another object of the present invention is to provide a wideband radar ranging system with a measurable center frequency and jam detection.
Still another object of the present invention is to provide a radar having a single transmit/receive oscillator and simplified STC for low cost applications.